The present invention relates to a pneumatic tire whose tread portion is provided with sipes, more particularly to a design of a pair of sipes capable of preventing cracks at the bottoms of the sipes and improving demolding of the vulcanization mold with siping blades.
Pneumatic tires are often provided in the tread portion with sipes in order to adjust the rigidity of ground contacting tread elements such as block, rib and the like, or increase edge effect for improving the traction performance and braking performance on icy roads for example. If such sipped tire receives a force to open the sipes during rolling and as a result stress concentrates at the bottoms of the sipes, there is a possibility that cracks occur at the bottoms of the sipes.
In Japanese Patent Application Publication No. H3(1991)-007604, in order to prevent cracks, a sipe (a) is provided at the bottom with a circular cross sectional shape (c) whose diameter is larger the thickness (w) of the sipe as shown in FIG. 5(a).
If such sipes (a) are formed at small intervals by the use of siping blades (g) provided on the inside of a vulcanizing mold, then it becomes hard to demold or the rubber (ba) between the siping blades (g) is torn off by the siping blades (g) when demolding as shown in FIG. 5(b).